1. Field of the Invention
The present invention relates to a cross-linked hydrogel and the preparation a hydrogel, a solution for the preparation of a hydrogel and a hydrogel sheet.
2. Background of the Invention
Free radical, network-producing polymerisations are used in a variety of applications including coatings, information storage systems, films, and aspherical lenses and biomaterials.
Cross-linked hydrogels are commonly prepared by free radical polymerisation. Over the past three decades, a number of hydrogels differing in structure, composition, and properties have been developed. Hydrogels are insoluble, water-swollen networks composed of hydrophilic homo-or copolymers. They are desirable for biomedical applications because of their high water content and rubbery nature, similar to natural tissue.
Free radical polymerisation processes are initiated by free radical initiators to obtain ensure proper polymerisation rates. These free radical initiators are activated by irradiation e.g. in the form of E-beam, microwaves, gamma or light (which includes UV, visible or near infrared). Other methods of initiating free radical polymerisation are thermal initiation and redox initiation.
While all initiation methods have their advantages/disadvantages the use of photopolymerisation is recognized as a fast, convenient and controllable way of preparing hydrogels through free radical polymerisation. The polymerisation process can be carried out under ambient or physiological conditions and even in the presence of biologically active materials. There are other advantages of using the photopolymerisation technique for biomaterials. In general, the process is benign and the process may also proceed rapidly at ambient conditions for most monomers and conventional initiators, i.e. fast curing rates. In addition, the ability to direct exposure of for example UV light and time of incidence to achieve spatial and temporal control is particularly advantageous for the formation of complex devices.
Due to their biocompatibility, permeability, and physical characteristics, hydrogels are suitable for use in many medical applications, including tissue engineering. Hydrogels may be useful for manipulation of tissue function or for scaffolds for tissue regeneration or replacement. The use of photopolymerisation in the preparation of hydrogels is advantageous in comparison with conventional cross-linking methods because liquid hydrogels precursors can be delivered and cross-linked to form hydrogels in situ in a minimally invasive manner. This process also renders it possible to achieve spatial and temporal control over the conversion of a liquid to a gel, so that complex shapes can be fabricated. Hydrogels can be formed with varying polymer formulations in three-dimensional patterns since sequentially polymerised layers will firmly adhere to one another.
Photopolymerised hydrogels can be designed to degrade via hydrolytic or enzymatic processes and can be modified with biofunctional moieties within their structure to manipulate cell behaviour and to generate organ-specific tissue formation. These photopolymerisable hydrogels can be used as barriers, localized drug delivery depots, cell encapsulation materials, and scaffold materials. Other biomedical applications include the prevention of thrombosis, post-operative adhesion formation, drug delivery, coatings for biosensors, guide-wires and catheters, and for cell transplantation.
Visible or UV light can interact with light sensitive compounds called photo-initiators to create free radicals that can initiate polymerisation to form cross-linked hydrogel (3-D polymeric networks). In vivo this principle has been utilized to polymerise or cure materials in dentistry to form sealant and dental restorations in situ. Photopolymerisations has also been used in electronic materials, printing materials, optical materials, membranes, polymeric materials, and coatings and surface modifications.
Photopolymerisation has several advantages over conventional polymerisation techniques. These include spatial and temporal control over polymerisation, faster curing rates (less than a second to a few minutes) at room or physiological temperature, and minimal heat production. Furthermore, photopolymerisation can be utilized to create hydrogels in situ from aqueous precursors in a minimal invasive manner. Fabrication of polymers in situ is attractive for a variety of biomedical applications because this allows one to form complex shapes that adhere and conform to tissue structures, for example laparascopic devices, catheters, or subcutaneous injection with trans-dermal illumination.
Polymerisation conditions for in vivo applications are however difficult since biological systems require a narrow range of acceptable temperatures and pH, as well as absence of toxic materials such as monomers and organic solvents is demanded. Some photopolymerisations systems may overcome these limitations because the polymerisation conditions are sufficiently mild (low light intensity, short irradiation time, physiological temperature, and low organic solvent levels) to be carried out in the presence of cells and tissues.
Photopolymerisation schemes generally use a photoinitiator that has a high absorption at a specific wavelength of light to produce radical initiating species. Other factors that should be considered include its biocompatibility, solubility in water, stability, and cytotoxicity. Various photoinitiators have been investigated to achieve better photopolymerisation. Photoinitiation is classified in three major classes depending on the mechanism involved in photolysis. The classes are radical photopolymerisation trough 1) photo-cleavage, 2) hydrogen abstraction and 3) cationic photopolymerisation. Cationic photoinitiators are generally not utilized in tissue engineering applications because they generate protonic acids and toxic side products. Cationic photopolymerisation will not be discussed further here.
In radical photopolymerisation by photocleavage, the photoinitiators undergo cleavage at C—C, C—Cl, C—O, or C—S bonds to form radicals when exposed to light. Water-soluble photoinitiators include aromatic carbonyl compounds such as benzoin derivatives, benziketals, acetophenone derivatives, and hydroxyalkylphenones. Acetophenone derivatives that contain pendant acrylic groups have been shown to substantially reduce the amount of unreacted photoinitiator with no significant loss in the initiation efficiency. Acetophenone derivatives, such as, 2,2-dimethoxy-2-phenyl acetophenone, have been used as photoinitiators to form hydrogels from acrylated polyethylene glycol (PEG) derivatives in several biomaterial studies.
Radical photopolymerisation by hydrogen abstraction: When subjected to UV irradiation, photoinitators such as aromatic ketones (i.e., benzophenone and thioxanthone) undergo hydrogen abstraction from an H-donor molecule to generate a ketyl radical and a donor radical. The initiation of photopolymerisation usually occurs through the H-donor radical while the ketyl radical undergoes radical coupling with the growing macromolecular chains. The photoinitiator propyl thixanthone has been shown to be cytocompatible.
Effective photoinitiators are for example compounds such as benzophenone, acetophenone, fluorenone, benzaldehyde, propiophenone, anthraquinone, carbazol, 3 or 4-methylacetophenone, 3 or 4-methoxybenzophenone, 4,4′-dimethoxybenzophenone, allylacetophenone, 2,2′-diphenoxyacetophenone, benzoin, methylbenzoin ether, ethylbenzoin ether, propylbenzoin ether, benzoin acetate, benzoinphenyl carbamate, benzoin acrylate, benzoinphenyl ether, benzoyl peroxide, dicumyl peroxide, azo isobutyronitrile, phenyl disulphide, acyl phosphene oxide or chloromethyl anthraquinone as well as mixtures thereof.
Peroxy—compounds, i.e. compounds containing an —O— binding, where oxygen has the oxidation number −1 are known as strong oxidation agents. They are capable of producing free radicals in many environments. As such peroxy-compounds have been utilized in free radical polymerisations as initiators of various kind, i.e. thermal, photo or redox initiation.
Persulphate (peroxydisulphate) is well known as an initiator of vinyl polymerisation in aqueous systems. Often used as a thermal initiator, where thermal decomposition produces radical ions, which directly or indirectly cause chain propagation. Peroxides may also be used as photoinitiators of vinyl polymerisation processes; both hydrogen peroxide, peroxydisulphate and peroxydiphosphate have been utilized for this purpose. The reaction scheme for the initiation of peroxydisulphates by photodecomposition is similar to that of thermal initiation. From the reaction scheme it is evident that the sulphate or hydroxyl radicals or a combination thereof may initiate polymerisation.
The peroxydisulphate is decomposed into sulphate ion radicals. These radicals are capable of reacting with a macromer or monomer unit (denoted M) to create a macromer or monomer radical. Furthermore the sulphate ion radical is capable of hydrogen abstraction from water thus creating hydroxyl radicals, which may react with a macromer or monomer unit creating another macromer or monomer radical.S2O8−+(ΔOR hv)→2SO4●−SO4●−+M→−SO4M.SO4●−+H2O→HSO4−+HO.HO.+M→HOM.
It is also well known that decomposition can be induced by the addition of reducing agents such as ferrous ions:Fe2++S2O82−→Fe3++SO42−+SO4●−
Peroxydisulphates have commonly also been employed in irradiation polymerisation processes where irradiation with γ rays are used. Another process concerned with the chemistry of peroxides has proven useful in free radical polymerisation, namely the photo-Fenton reaction. The photo-Fenton reaction has been largely applied in oxidative degradation of organic pollutants for water treatment and in some special cases depolymerisation technique. The photo-Fenton reaction has also been described to produce polymers from vinylpyrrolidone (VP) and copolymers hereof (copolymers of VP and MAA (methacrylic acid).
The photo-Fenton reaction is a process comprising two-interconnected steps. Firstly, hydrogen peroxide is decomposed into hydroxyl radicals by the presence of Fe2+, which is oxidized to Fe3+. In the dark the reaction is retarded after complete conversion of Fe2+to Fe3+. Irradiation of the system by UV-light (around 365 nm) results in photoreduction of Fe3+ to Fe2+, which produce new hydroxyl radicals with hydrogen peroxide according to the first process or to an additional effect of direct peroxide photolysis. In the above mentioned polymerisation process practically no polymerisation occurred without light. Hence, to create a high enough concentration of hydroxyl radicals to initiate chain propagation, light is necessary.
It is believed, that any free radical initiation system, especially free radical polymerisations carried out in aqueous solutions capable of generating soluble peroxides may be greatly enhanced by the addition of soluble metal ions capable of initiating the decomposition of the formed peroxides (redox process). These metal ions include iron and other transition metals having at least to readily available oxidation states.
Polymerisation of monomers using visible or UV irradiation has been thoroughly investigated. While such systems may work well for many applications including many biomaterials, they generally cannot be utilized in tissue engineering because most monomers are cytotoxic. As a result, photopolymerisable hydrogels for tissue engineering have generally been formed from macromolecular hydrogel precursors. Such precursors are water-soluble polymers with two or more reactive groups. Examples of photopolymerisable macromers include PEG acrylate derivatives, PEG methacrylates derivatives.
Poly(ethylene glycol) is a non-toxic, water soluble polymer which resists recognition by the immune system. The term PEG is often used to refer to polymer chains with molecular weights below 20.000, while poly(ethylene oxide) (PEO) refers to higher molecular weight polymers. PEG may transfer its properties to another molecule when it is covalently bound to said molecule. This may result in toxic molecules becoming non-toxic (as is the case with PEG-DMA which is non-toxic pegylated dimethacrylic acid) or hydrophobic molecules becoming soluble when coupled to PEG. It exhibits rapid clearance from the body, and has been approved for a wide range of biomedical applications. Because of the properties, hydrogels prepared from PEG are excellent candidates as biomaterials.
Polyvinyl alcohol (PVA) derivatives, and modified polysaccharides such as hyaluronic acid derivatives and dextran methacrylate have been described as useful macromolecular precursors.
Polyvinylpyrrolidone (PVP) is another useful candidate. Polymeric materials based on poly(N-vinyl-2-pyrrolidone) (PVP) and its copolymers have found intense applications as hydrogels and membranes used in drug delivery systems, adhesive formulations, and in photographic and lithographic coatings. The low chemical toxicity of PVP, its solubility in water and in organic solvents as well as its ability to complex with many kind of substrates like dyes, surfactants, and other polymers, have promoted its use as a protective colloid in pharmaceutical and cosmetic products.
3. Description of the Related Art
U.S. Pat. No. 5,410,016 discloses the development of photopolymerisable biodegradable hydrogels. The hydrogel comprises a macromer on which is grafted biodegradable units such as poly(alpha-hydroxy acid), poly(glycolic acid), poly(DL-lactic acid) and poly(L-lactic acid). Other useful materials includes poly(amino acids), poly(anhydrides), poly(orthoester), poly(phosphazines) or poly(phosphoester). Polylactones like poly(ε-caprolactone), poly(δ-valerolactone) or poly(λ-butyrolactone).
PVP is mentioned as a possible water-soluble region of the macromer. Acrylates, diacrylates, oligoacrylates, methacrylates, dimethacrylates, oligomethacrylates are mentioned as polymerisable regions of the macromer. The macromers are synthesized in organic solvents and photosensitive macromers prepared from these macromers. A combination of PEG-DMA and PVP is mentioned, the photoinitiators employed are commonly known. Peroxydisulphates may alternatively be used as thermal initiators.
U.S. patent application No. 2001/0044482 discloses interpenetrating polymer network (IPN) compositions and a process for the manufacturing of hydrogel contact lenses. The polymeric material is prepared by polymerisation of an unsaturated alkyl(meth)acrylate or its derivatives such as 2-hydroxyethyl methacrylate (HEMA) as the principle monomer, optionally vinyl containing comonomer(s) to enhance the resulting water absorbing capability, polymerisable multi-functional cross-linking agent(s), an irradiation initiator and/or thermal initiator, optionally other additives to impart the resulting hydrogel specific properties such as UV-blocking ability and handling colorants; in the presence of a soluble hydrophilic interpenetrating networking agent such as polyvinyl-pyrrolidone or poly-2-ethyl-2-oxazoline (PEOX) with a specific molecular weight. PVP is mentioned as IPN agent, PEG-DMA is mentioned as a cross-linker and photoinitiation or/and thermal polymerisation is mentioned. UV or thermal initiation is used alone or in combination.
The hydrogels are prepared by mixing all the ingredients (dissolved in each other), subjecting the mixture to a short UV curing (minutes) followed by a longer thermal post curing (hours). The obtained dry gel is then hydrated after curing. The method of preparation is used in order to obtain a thorough curing process to secure that all monomers have been consumed in the curing process. The curing process is quite time consuming.
U.S. Pat. No. 5,005,287 discloses a process for forming and applying a hydrophilic coating cured by UV-light to a plastic or metal part either directly, or indirectly via plastic film, to safety razor or razor blade unit. The coating comprises a water-soluble polymer or copolymer of PVP, at least one radically polymerisable vinyl monomer and a photoinitiator. Several vinylic monomers, mostly of the type acrylic acid or methacrylic acids are mentioned. Oligoethylene glycol bisacrylate is mentioned as a suitable cross-linker. A wide range of photoinitiators is mentioned. Water is mentioned as a polymerisation solvent. The cured polymer layers are of 5-1000 μm thickness. Curing times are in seconds to minutes.
W. K. Wilmarth and A. Haim in J. O. Edwards (ed.), Peroxide reaction mechanisms, Wiley-Interscience, New York, 1962, pp. 175-225, discloses the reactions of peroxidisulphate with various substrates in aqueous solution from a mechanistic viewpoint. The thermal and photolytic decompositions of the peroxidisulphate ion are described in detail.
C. G. Roffey in JOCCA 1985 (5), 116-120, discloses the photodecomposition of the peroxydisulphate ion in water or water/ethanol mixtures producing sulphate ion radicals, which are potentially useful in various emulsion polymerisation techniques.
S. Lenka and P. L. Nayak in Journal of Photochemistry, 1987, 36, 365-372, disclose the use of peroxydiphosphate to photopolymerise methyl methacrylate.
In-Sook Kim et al, in Arch. Pharm. Res. 2001, 24, No. 1 69-73 discloses the use of ammonium peroxydisulphate and UV light in vinylic radical polymerisation of a biodegradable hydrogel formed from a functionalised dextran (glycidyl methacrylate dextran and dimethacrylate poly(ethylene glycol). The photopolymerisation process is carried out with a rather high amount of ammonium persulphate (10% of polymer weight) and a very long UV-curing time of 80 minutes.
Thus, there is still a need for a hydrogel, which can be produced in a fast and simple manner, being non-toxic and producible in both thin and thick layers. Surprisingly, such a hydrogel has been achieved by the present invention.